The invention relates to microchannel plate (MCP) based detectors. In particular, the invention pertains to MCP-based detectors of reduced size and simplified manufacture having flexible electrical leads. The invention is especially useful in detectors for scanning electron microscopes (SEM), but is also useful in detectors for other applications.
MCP-based detectors are becoming widely accepted in SEM imaging applications requiring low beam energies and currents. Images have been obtained of uncoated dielectric samples without the usual charging effects and of semiconductor materials with little or no damage to surfaces which are sensitive to radiolytic effects. Useful images have been obtained with beam energies as low as 200 volts and currents as low as 1 picoamp.
The majority of MCP detectors for high gain applications are constructed by stacking a series of ceramic insulators, metal electrical contacts, MCPs and anodes in a specific configuration to achieve the proper spacing and alignment necessary for a functional detector. FIGS. 1A and 1B illustrate typical arrangements of known microchannel plate detectors. In FIG. 1A, the detector 10 has an aperture 11 through which an electron beam 12 is directed at a sample S. Back-scattered and secondary electrons 12' are detected. The detector 10 is a stacked arrangement including opposed housing elements 13 and 14, insulator 16, metal contact tab assemblies 18, anodes 20 and a pair of MCPs, namely input MCP 22A and output MCP 22B, in abutting relation having respective channels 24A and 24B disposed at an angle with respect to the detector axis 26 to form a chevron structure for reducing ion feedback. A shielded wiring harness 27 containing high voltage leads and anode leads 28 is employed to reduce noise. In particular, the anode leads are coaxial cables and an outer braided sheath 30 is employed as a ground.
FIG. 1B illustrates a MCP detector 32 which is similar to the arrangement of FIG. 1A except that the respective input and output MCPs 34A and 34B do not have a central beam aperture and the detector does not utilize a wiring harness.
As is well known to those skilled in the art, all MCP-based detectors operate in high vacuum (10.sup.-5 torr or better) and require a high voltage potential across the MCPs to function. A typical chevron arrangement employing 2 MCPs (FIG. 1B) used to detect electrons would have the input face 35A of the input MCP 34A grounded, the output face 36B of the output MCP 34B biased at +2000 volts, and the anode 34 biased at +2200 volts. Thus, compatibility with a high voltage, high vacuum environment is essential for MCP-based detectors to be acceptable.
The devices illustrated in FIGS. 1A and 1B are relatively large being at least 5.2 mm and 7.0 mm thick and 25.4 mm and 50.8 mm in diameter respectively. The size of currently available MCP-based detectors causes difficulties when samples are manipulated in a SEM. The detector thickness limits the working distance, i.e., the distance between the bottom of the electron gun lens and the sample, and therefore the image resolution of the SEM; the diameter interferes with the sample when high tilt angles are used, as required in many semiconductor inspection procedures.
Reduction in the size of a MCP-based detector is thus desirable. However, it is important to consider the effect of the dimension on the electrical characteristics of the detector, so that, currently acceptable noise and sensitivity levels are met or exceeded.
Each layer in the detector has a separate function, and one or more assorted small parts are required for each layer of the assembly. The total number of parts required to assemble a conventional MCP-based detector is high. For example, thirty-one (31) parts are required for the detector 10, shown in FIG. 1A. Assembly of the device and the shielded harness is difficult and time consuming. Also, each of the many parts is made to high tolerance. Thus, the inventory required to achieve normal production is costly to maintain and control.
A simplified MCP-based detector is therefore desired having a smaller size and a lower number of parts. Also, a detector assembly is desired which is more readily adaptable to different instruments and applications and has a greater sensitivity and versatility.